The present invention relates to a high-aluminum-containing ferritic stainless steel which is used at high-temperature such as a catalyst in an emission control system for motor vehicles, a heating device, and others.
The high-aluminum ferritic stainless steel has a high oxidation resistance at high temperature so that it is extensively used for materials in heating devices such as a stove pipe, and electric heater elements.
Ceramics have heretofore been used as a catalyst carrier of a catalytic converter provided in an emission control system for motor vehicles, and ferritic stainless steel is now beginning to be used instead. One of the defects of the ceramic is that the ceramic is vulnerable to thermal shock. Another is that the ceramic has a high heat capacity. Consequently, when the ceramic is used for a catalyst carrier, a long time is required to be heated to a catalyst reacting temperature. A metallic carrier made of high-aluminum ferritic stainless steel has not such defects as the ceramic.
The stainless steel is formed into a sheet of foil having a thickness of about 50 .mu.m to be used as a catalyst carrier for catalytic converters. Since abnormal oxidation is liable to occur on the foil and the converter is used in an atmosphere of exhaust gases which is severe oxidization conditions, the foil must have extremely resistive characteristics against high-temperature oxidation. To meet the requirements, a high-aluminum-containing ferritic stainless steel including 20Cr-5Al as a base, rare-earth elements and yttrium is used. However, such a stainless steel does not have sufficient high-temperature oxidation resistance, so that an extended use of the steel causes abnormal oxidation.
In addition, from the point of global warming and public nuisance, stricter limits have been imposed on emission control. To comply with the standards, it is necessary to quickly heat the catalyst to the catalyst reacting temperature after the start of the engine. To this end, the temperature of exhaust gas is elevated, or the converter is positioned directly under an exhaust manifold. On the other hand, the power of the engine tends to be increased in recent years, which causes also increase of the temperature of the exhaust. These conditions enhance oxidation and corrosion of the catalyst carrier. Therefore, it is imperative to provide a high-aluminum-containing ferritic stainless steel having improved high-temperature oxidation resistance.
In order to improve the resistance of the high-aluminum-containing ferritic stainless steel against high-temperature oxidation, contents of chrominum, aluminum, rare earth elements and yttrium are increased, which is disclosed in Japanese Patent Application Laid-Open 63-45351. However, slabs and hot-rolled plates made of such a stainless steel are poor in toughness and hence, in productivity. More particularly, increase in chromium and aluminum contents causes not only rise in the cost of the component material, but also decreases productivity due to poor toughness. As a consequence the yield of the stainless steel decreases, thereby extremely increasing the manufacturing cost, or the production becomes altogether impossible. Alternatively, the rare-earth elements and yttrium are added to promote high-temperature oxidation resistance. However, excessive quantity of these elements rather decreases the high-temperature oxidation resistance, and causes deterioration in toughness.
In addition, the ferritic stainless steel foil of the catalytic converter is subjected to heat cycles where heating and cooling are repeated, thereby causing deformation of the foil. Thus, the catalyst carrier must have high heat resistance.